When connecting fluid-conducting duct elements, especially in ventilation duct systems, it is important that the joints between the tubular elements be sufficiently sealed. U.S. Pat. No. 3,955,834 disclosed various modes of providing sealing means in the joint between a tube connector inserted in and interconnecting two tubes. FIGS. 3-9 and of this U.S. patent of 1976 show how an external, circumferential, flexible sealing ring having L-shaped cross-section is mounted at the end of a tubular element. The sealing ring is placed with its base abutting the outer surface of the tubular element, whereupon the free end portion of the tubular element is bent rearwardly outwards and turned back towards the wall of the tubular element. The sealing ring has a sealing lip radially extending away from the base and adapted to seal against an inner surface of an outer tube, in which the tubular element is inserted (see FIG. 8).
This basic method of mounting the sealing ring at the end of a tubular element is still used in practice, as is shown in the Swedish-language catalogue "Veloduct.RTM., Cirkulart kanalsystem" issued in 1997 by ABB Ventilation Products AB, Division Veloduct. On page 2 of this catalogue, there is shown how a sealing ring with L-shaped cross-section is fastened by the bonding and clamping operation described above. This known technique, which is schematically shown in FIG. 1 of the appended drawings, has however several disadvantages which will be related below.
The base of the L-shaped sealing ring is placed on an annular, circumferential step which protrudes radially outwards. This step leads to various drawbacks related to the mounting of the sealing ring as well as the structure of the sealing ring and its manufacture. When mounting the sealing ring on the tubular body, it is difficult to put the base on the raised step. Normally, this is a manual operation and the person mounting the ring often has to adjust the sealing ring on the raised step before the bending operation. Due to the raised step, the sealing ring has a tendency to slide inwardly on the tubular body during the bending of the end portion of the tubular body. This sliding movement may jeopardize the bending and clamping operation and thereby prevent safe fastening of the sealing ring. The bulge on the front end of the sealing ring base is normally not enough to prevent this sliding, especially in cases where the sealing ring material is relatively soft.
In practice, it has also been found that this known L-shaped sealing ring has special drawbacks for tubes of a large diameter, say exceeding about 300 mm. For tubular elements of such a large diameter, the radial dimension of the sealing lip is larger than the axial dimension of the base. Such sealing rings have a tendency to "tilt" about 90.degree., which means that the sealing ring is abutting the wall of the tubular body whereas the sealing ring base is directed radially outwards from the tubular wall. This misplacement has to be corrected manually in mounting.
The bulge at the free end of the sealing ring base makes it difficult to produce the sealing ring which consists of rubber. Due to the bulge, special tooling has to be used which involve specific drawbacks. The bulge makes it hard to achieve proper release from the mould. Especially when the sealing ring is injection moulded, the bulge is disadvantageous.
Thus, there is a need for a tubular element having improved sealing means, as well as a new method for manufacturing such a tubular element.